[mlgo] fix build rules

This was prompted by D95727, which had the side-effect to break the
'release' mode build bot for ML-driven policies. The problem is that now
the pre-compiled object files don't get transitively carried through as
'source' anymore; that being said, the previous way of consuming them
was problematic, because it was only working for static builds; in
dynamic builds, the whole tf_xla_runtime was linked, which is
undesirable.

The alternative is to treat tf_xla_runtime as an archive, which then
leads to the desired effect.

Differential Revision: https://reviews.llvm.org/D99829